The turbojet or low-bypass turbofan engine provides a large amount of jet thrust but at a relatively high fuel penalty. Turbofan embodiments, wherein a ducted fan is used to direct bypass propulsive air around the periphery of the jet core, provide good efficiency at cruise velocities but poor performance at relatively high or low speeds. Further, propeller based propulsion (to include, turbo props and un-ducted propfans) allows for reduced fuel consumption when an airframe is traveling at fairly low speeds.
As a general proposition, an engine is most efficient when the exit velocity of the propulsive system is closely matched to the forward motion of the airframe. In the previously noted examples, each engine excels at a specific purpose and they are tailored for flight conditions pertaining to that purpose. However, many military airframes have a spectrum of mission profiles in which it would be advantageous to possess both the increased mass flow exit velocity of a turbojet engine during one mission leg and improved fuel economy and propulsive efficiency during another leg.
Therefore, a need exists for an improved dual mode turbofan engine capable of selectively switching between turbofan and prop driven configurations in response to operating conditions, environmental conditions, or operator selection.